Description
The present invention relates to amplifiers.
It is often necessary for the gain of an amplifier to be stabilized so that the gain does not vary with environmental factors such as temperature and manufacturing tolerances.
A circuit for stabilizing the bias current of a bipolar amplifier is disclosed in EP-A-0942524. The disclosed circuit has a driver transistor which controls the base biasing currents of a plurality of power transistors, connected in series, and a control transistor via respective base resistors. A differential amplifier has one input connected to the output of a potential divider, for setting the desired bias point, and the other input connected to the collector of the control transistor. The collector of the control transistor is also connected to a regulated supply voltage via a resistor. One of the outputs of the differential amplifier is coupled to the base of the driver transistor. Any increase in the collector currents of the power transistors is reflected in the collector current of the control transistor thereby reducing the voltage fed to the second input of the differential amplifier. Consequently, the voltage applied to the base of the driver transistor is reduced and the base biasing currents of the power transistors and the control transistor are reduced. Thus, the bias currents of the power transistors are controlled by a negative feedback control loop.
It can be seen that the output of the differential amplifier cannot be usefully less than two Vbe""s plus the voltage across the base resistors of the power transistors. In the case of GaAs HBT devices, Vbe is 1.2V, leaving less than 0.6V available for the output range of the differential amplifier when Vcc is 3V which is typical.
Furthermore, the prior art designs are less than ideal for Enhanced Data Rates for GSM (xe2x80x9cEDGExe2x80x9d) systems.
According to the present invention, there is provided an amplifier circuit comprising:
a first common emitter transistor amplifier including a base biasing resistor;
a second common emitter transistor amplifier including a base biasing resistor with bias collector current of the second common emitter transistor amplifier being substantially proportional to that of the first common emitter transistor amplifier; and
a differential amplifier having one input connected to a reference voltage and a second input connected to receive the output of the second common emitter transistor amplifier,
wherein the base biasing currents of the transistors of the common emitter transistors amplifiers are provided from an output of the differential amplifier which is directly coupled to the base biasing resistors of the first and second common emitter amplifiers.
The first common emitter transistor amplifier may comprise a plurality of transistors having respective base biasing resistors and sharing a common collector load impedance.
Preferably, the output of the second common emitter amplifier is taken from the junction between two series connection collector load resistors.
A bias control input may be included to provide an additional collector current path, for example via a resistor, for the second emitter follower amplifier. The bias-control input can be used to control the gain of the first emitter follower amplifier, for instance to give an output signal a desired power envelope.
An amplifier circuit according to the present invention may be employed advantageously in a mobile phone.